Along with the greatly increased use of computers for offices and for manufacturing facilities, there has developed a need for a cable which may be used to connect peripheral equipment to mainframe computers and to connect two or more computers into a common network. Of course, the sought-after cable desirably should provide substantially error-free transmission at relatively high rates.
A number of factors must be considered to arrive at a cable design which is readily marketable for such uses. The jacket of the sought-after cable should exhibit low friction to enhance the pulling of the cable into ducts or over supports. Also, the cable should be strong, flexible and crush-resistant, and it should be conveniently packaged and not unduly weighty. Because the cable may be used in occupied building spaces, flame retardance also is important.
The sought-after data transmission cable should be low in cost. It must be capable of being installed economically and be efficient in terms of space required. It is not uncommon for installation costs of cables in buildings, which are used for interconnection, to outweigh the cable material costs. Building cables should have a relatively small cross-section inasmuch as small cables not only enhance installation but are easier to conceal, require less space in ducts and troughs and wiring closets and reduce the size of associated connector hardware.
Also important to cost is whether or not the system is arranged to provide transmission in what is called a balanced mode, In balanced mode transmission, voltages and currents on the conductors of a pair are equal in amplitude but opposite in polarity. This requires the use of additional components, such as transformers, for example, at end points of the cable between the cable and logic devices, thereby increasing the cost of the system. Oftentimes, computer equipment manufacturers have preferred the use of systems characterized by an unbalanced mode to avoid investing in additional components for each line. In an unbalanced mode transmission system, voltages and currents on the conductors of a pair are not characterized by equality of amplitude and opposition of polarity. Computer equipment manufacturers have adopted unbalanced mode transmission and are not prone to retrofit their equipment in order to simplify the interconnection of peripherals. At the same time, however, peripheral connection arrangements must meet predetermined attenuation and crosstalk requirements.
To satisfy present, as well as future needs, the sought-after cable should be capable of suitable high frequency data transmission. This requires a tractable loss for the distance to be covered, and crosstalk and electromagnetic interference (EMI) performance that will permit substantially error-free transmission. Also, the cable must not contaminate the environment with electromagnetic interference.
Of importance to the design of local area network copper conductor cables are the speed and the distances over which data signals must be transmitted. In the past, this need has been one for interconnections operating at data speeds up to 20 kilobits per second and over a distance not exceeding about 150 feet. This need has been satisfied in the prior art with single jacket cable which may comprise a plurality of insulated conductors that are connected directly between a computer, for example, and receiving means such as peripheral equipment.
In today's world, however, it becomes necessary to transmit data signals at much higher speeds over distances which may include several hundreds of feet. Currently, equipment is commercially available that can transmit 10 Mbps data signals for 300 or 400 feet. Even at these greatly increased distances, the transmission must be substantially error-free and at relatively high rates. Further advances in data rate/distance capability are becoming increasingly difficult because of crosstalk between the pairs of commerically available cables.
Often, this need has been filled with coaxial cable comprising the well-known center solid and outer tubular conductor separated by a dielectric material. The use of coaxial cables, which inherently provide unbalanced transmission, presents several problems. Coaxial connectors are expensive and difficult to install and connect, and, unless they are well designed, installed and maintained, can be the cause of electromagnetic interference. Of course, the use of coaxial cables does not require components such as transformers at each end to provide balanced mode transmission, but the size and connectorization of coaxial cables outweigh this advantage.
Faced with this situation, customers, local area network (LAN) vendors and distribution system vendors began exploring alternatives for making LAN wiring more affordable and manageable while still providing the necessary level of transmission performance. Overlooked by most investigators has been the unshielded twisted pair long used for premises distribution of telephone signals.
The unshielded twisted pair has always been used for telephone transmission in the balanced (differential) mode. Used in this manner, the unshielded twisted pair has excellent immunity to interference whether from the outside (EMI) or from signals on other pairs (crosstalk). Over the past several years, in fact, some LAN designers, have come to realize the latent transmission capability of unshielded twisted pair wire. Especially noteworthy is the twisted pair's capability to transmit rugged quantized digital signals as compared to corruptible analog signals.
The limitations imposed by crosstalk, especially near-end crosstalk, on the data rate/distance capabilities of twisted pair cables are generally recognized. Hereinafter, near-end cross talk is referred to as crosstalk. Often overlooked as a means of reducing crosstalk between twisted pairs is the pair twist scheme. For instance, one recent specification required simply that each pair be twisted with a twist length less than six inches. The most common treatment for crosstalk is to add shielding over each twisted pair to confine its electric and magnetic fields. However, as the electric and magnetic fields are confined, resistance, capacitance and inductance all change, each in such a way as to increase transmission loss. For instance, it is not unusual to find designs of shielded pairs whose attenuation is three times that of similar unshielded pairs. One company markets a cable in which each pair of conductors is provided with a shield and a braid is provided about the plurality of pairs. In order to compensate for the increased losses, the conductor insulation must be increased in thickness. As a result, the insulated conductors are bulky, expensive and cannot be terminated with conventional connector hardware.
Cable connectorability is very important and is more readily accomplished with twisted insulated conductor pairs than with any other medium. A widely used connector for insulated conductors is one which is referred to as a split beam connector. Desirably, the outer diameter of insulated conductors of the sought-after cable is sufficiently small so that the conductors can be terminated with such existing connector systems.
The sought after cabling arrangement should be one that may be used to replace the well known D-inside wiring which comprises a plurality of twisted insulated conductor pairs. The pairs are non-shielded and are enclosed in a jacket. Improved pair isolation has long been sought in such wiring to reduce crosstalk.
Seemingly, the solutions of the prior art to the problem of providing a local area network cabling arrangement which can be used to transmit, for example, data bits error-free at relatively high rates over relatively long distances have not yet been totally satisfying. What is needed and what is not provided by the prior art is a cable which is inexpensively made and which has crosstalk levels which are an order of magnitude lower than those experienced with D-inside wiring or with other economically available products. Further, any arrangement proposed as a solution to the problem should be one which does not occupy an undue amount of space and one which facilitates a simplistic connection arrangement. There is a need to provide cables that can transmit data rates of many megabits per second, error-free, from stations to closets or between computer cabinets separated by comparable distances to main rooms, be readily installed, fit easily into building architectures, and be safe and durable.